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Thursday, December 13, 2012

The LHC experiments

The LHC experiments

The six experiments at the LHC are all run by international
collaborations, bringing together scientists from institutes all over
the world. Each experiment is distinct, characterised by its unique
particle detector.
The two large experiments, ATLAS and CMS,
are based on general-purpose detectors to analyse the myriad of
particles produced by the collisions in the accelerator. They are
designed to investigate the largest range of physics possible. Having
two independently designed detectors is vital for cross-confirmation of
any new discoveries made.
Two medium-size experiments, ALICE and LHCb, have specialised detectors for analysing the LHC collisions in relation to specific phenomena.
Two further experiments, TOTEM and LHCf,
are much smaller in size. They are designed to focus on "forward
particles" (protons or heavy ions). These are particles that just brush
past each other as the beams collide, rather than meeting head-on.
The ATLAS, CMS, ALICE and LHCb detectors are installed in four
huge underground caverns located around the ring of the LHC. The
detectors used by the TOTEM experiment are positioned near the CMS
detector, whereas those used by LHCf are near the ATLAS detector.

ATLAS

ATLAS is one of two general-purpose detectors at the LHC. It will investigate a wide range of physics, including the search for the Higgs boson, extra dimensions, and particles that could make up dark matter. ATLAS will record sets of measurements on the particles created in collisions - their paths, energies, and their identities.
This is accomplished in ATLAS through six different detecting
subsystems that identify particles and measure their momentum and
energy.
Another vital element of ATLAS is the huge magnet system that bends the paths of charged particles for momentum measurement.
The interactions in the ATLAS detectors will create an
enormous dataflow. To digest these data, ATLAS needs a
very advanced trigger and data acquisition system, and a
large computing system.
More than 2900 scientists from 172 institutes in 37 countries work on the ATLAS experiment (December 2009).

Related links

How a detector works

The job of a particle detector is to record and visualise the explosions of particles that result from the collisions at accelerators.
The information obtained on a particle's speed, mass, and electric
charge help physicists to work out the identity of the particle.
The work particle physicists do to identify a particle that has
passed through a detector is similar to the way someone would study the
tracks of footprints left by animals in mud or snow. In animal prints,
factors such as the size and shape of the marks, length of stride,
overall pattern, direction and depth of prints, can reveal the type of
animal that came past earlier. Particles leave tell-tale signs in
detectors in a similar manner for physicists to decipher.
Modern particle physics apparatus consists of layers of
sub-detectors, each specialising in a particular type of particle or
property. There are 3 main types of sub-detector:

To help identify the particles produced in the collisions, the
detector usually includes a magnetic field. A particle normally travels
in a straight line, but in the presence of a magnetic field, its path
is bent into a curve. From the curvature of the path, physicists can
calculate the momentum of the particle which helps in identifying its
type. Particles with very high momentum travel in almost straight
lines, whereas those with low momentum move forward in tight spirals.

Tracking devices

Tracking devices reveal the paths of electrically charged
particles through the trails they leave behind. There are similar
every-day effects: high-flying airplanes seem invisible, but in certain
conditions you can see the trails they make. In a similar way, when
particles pass through suitable substances the interaction of the
passing particle with the atoms of the substance itself can be revealed.
Most modern tracking devices do not make the tracks of particles
directly visible. Instead, they produce tiny electrical signals that can
be recorded as computer data. A computer program then reconstructs the
patterns of tracks recorded by the detector, and displays them on a
screen.
They can record the curvature of a particle's track (made in the
presence of a magnetic field), from which the momentum of a particle
may be calculated. This is useful for identifying the particle.
Muon chambers are tracking devices used to detect muons. These
particles interact very little with matter and can travel long
distances through metres of dense material. Like a ghost walking through
a wall, muons can pass through successive layers of a detector. The
muon chambers usually make up the outermost layer.

Calorimeters

A calorimeter measures the energy lost by a particle that goes
through it. It is usually designed to entirely stop or ‘absorb’ most of
the particles coming from a collision, forcing them to deposit all of
their energy within the detector.
Calorimeters typically consist of layers of ‘passive’ or
‘absorbing’ high–density material (lead for instance) interleaved with
layers of ‘active’ medium such as solid lead-glass or liquid argon.
Electromagnetic calorimeters measure the energy of light
particles – electrons and photons – as they interact with the
electrically charged particles inside matter.
Hadronic calorimeters sample the energy of hadrons (particles
containing quarks, such as protons and neutrons) as they interact with
atomic nuclei.
Calorimeters can stop most known particles except muons and neutrinos.

Particle identification detectors

Two methods of particle identification work by detecting radiation emitted by charged particles:

Cherenkov radiation: this is light emitted when a charged
particle travels faster than the speed of light through a given medium.
The light is given off at a specific angle according to the velocity of
the particle. Combined with a measurement of the momentum of the
particle the velocity can be used to determine the mass and hence to
identify the particle.

Transition radiation: this radiation is produced by a fast
charged particle as it crosses the boundary between two electrical
insulators with different resistances to electric currents. The
phenomenon is related to the energy of a particle and distinguishes
different particle types.

How a detector works

The job of a particle detector is to record and visualise the explosions of particles that result from the collisions at accelerators.
The information obtained on a particle's speed, mass, and electric
charge help physicists to work out the identity of the particle.
The work particle physicists do to identify a particle that has
passed through a detector is similar to the way someone would study the
tracks of footprints left by animals in mud or snow. In animal prints,
factors such as the size and shape of the marks, length of stride,
overall pattern, direction and depth of prints, can reveal the type of
animal that came past earlier. Particles leave tell-tale signs in
detectors in a similar manner for physicists to decipher.
Modern particle physics apparatus consists of layers of
sub-detectors, each specialising in a particular type of particle or
property. There are 3 main types of sub-detector:

To help identify the particles produced in the collisions, the
detector usually includes a magnetic field. A particle normally travels
in a straight line, but in the presence of a magnetic field, its path
is bent into a curve. From the curvature of the path, physicists can
calculate the momentum of the particle which helps in identifying its
type. Particles with very high momentum travel in almost straight
lines, whereas those with low momentum move forward in tight spirals.

Tracking devices

Tracking devices reveal the paths of electrically charged
particles through the trails they leave behind. There are similar
every-day effects: high-flying airplanes seem invisible, but in certain
conditions you can see the trails they make. In a similar way, when
particles pass through suitable substances the interaction of the
passing particle with the atoms of the substance itself can be revealed.
Most modern tracking devices do not make the tracks of particles
directly visible. Instead, they produce tiny electrical signals that can
be recorded as computer data. A computer program then reconstructs the
patterns of tracks recorded by the detector, and displays them on a
screen.
They can record the curvature of a particle's track (made in the
presence of a magnetic field), from which the momentum of a particle
may be calculated. This is useful for identifying the particle.
Muon chambers are tracking devices used to detect muons. These
particles interact very little with matter and can travel long
distances through metres of dense material. Like a ghost walking through
a wall, muons can pass through successive layers of a detector. The
muon chambers usually make up the outermost layer.

Calorimeters

A calorimeter measures the energy lost by a particle that goes
through it. It is usually designed to entirely stop or ‘absorb’ most of
the particles coming from a collision, forcing them to deposit all of
their energy within the detector.
Calorimeters typically consist of layers of ‘passive’ or
‘absorbing’ high–density material (lead for instance) interleaved with
layers of ‘active’ medium such as solid lead-glass or liquid argon.
Electromagnetic calorimeters measure the energy of light
particles – electrons and photons – as they interact with the
electrically charged particles inside matter.
Hadronic calorimeters sample the energy of hadrons (particles
containing quarks, such as protons and neutrons) as they interact with
atomic nuclei.
Calorimeters can stop most known particles except muons and neutrinos.

Particle identification detectors

Two methods of particle identification work by detecting radiation emitted by charged particles:

Cherenkov radiation: this is light emitted when a charged
particle travels faster than the speed of light through a given medium.
The light is given off at a specific angle according to the velocity of
the particle. Combined with a measurement of the momentum of the
particle the velocity can be used to determine the mass and hence to
identify the particle.

Transition radiation: this radiation is produced by a fast
charged particle as it crosses the boundary between two electrical
insulators with different resistances to electric currents. The
phenomenon is related to the energy of a particle and distinguishes
different particle types.

Missing Higgs

A major breakthrough in particle physics came in the 1970s when
physicists realized that there are very close ties between two of the
four fundamental forces – namely, the weak force and the electromagnetic
force. The two forces can be described within the same theory, which
forms the basis of the Standard Model. This ‘unification’ implies that
electricity, magnetism, light and some types of radioactivity are all
manifestations of a single underlying force called, unsurprisingly, the
electroweak force. But in order for this unification to work
mathematically, it requires that the force-carrying particles have no
mass. We know from experiments that this is not true, so physicists
Peter Higgs, Robert Brout and François Englert came up with a solution
to solve this conundrum.
They suggested that all particles had no mass just after the Big
Bang. As the Universe cooled and the temperature fell below a critical
value, an invisible force field called the ‘Higgs field’ was formed
together with the associated ‘Higgs boson’. The field prevails
throughout the cosmos: any particles that interact with it are given a
mass via the Higgs boson. The more they interact, the heavier they
become, whereas particles that never interact are left with no mass at
all.
This idea provided a satisfactory solution and fitted well with
established theories and phenomena. The problem is that no one has ever
observed the Higgs boson in an experiment to confirm the theory. Finding
this particle would give an insight into why particles have certain
mass, and help to develop subsequent physics. The technical problem is
that we do not know the mass of the Higgs boson itself, which makes it
more difficult to identify. Physicists have to look for it by
systematically searching a range of mass within which it is predicted to
exist. The yet unexplored range is accessible using the Large Hadron
Collider, which will determine the existence of the Higgs boson. If it
turns out that we cannot find it, this will leave the field wide open
for physicists to develop a completely new theory to explain the origin
of particle mass.